Police suspect 'serial bomber' behind deadly Austin attacks

Four bombs that have gone off this month around Austin, Tex., have similarities that lead investigators to believe they are the work of a serial bomber, police said on Monday.

The latest bombing on Sunday injured two men and may have been activated by a trip wire, a more advanced design than the previous explosions that were set off when victims handled packages that were left on doorsteps. Those packages killed two earlier this month.

"We are clearly dealing with what we expect to be a serial bomber at this point," Austin Police Chief Brian Manley told a news conference. "We have seen similarities in the devices that exploded here last night and the other three devices."

The men, one aged 22 and the other 23, suffered non-life threatening injuries and were taken to the hospital on Sunday after they came upon a suspicious device on the side of a road in a residential neighbourhood, officials said.

Agents from the Federal Bureau of Investigation and U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives were at the scene, the FBI said on Twitter.

Investigators are trying to identify the person or people behind the three parcel bombs that exploded in three separate east side neighbourhoods, killing two African-American males and leaving a 75-year-old Hispanic woman fighting for her life.

The most recent blast happened in the southwestern Austin residential neighbourhood of Travis Country. The three previous bombings happened in residential neighbourhoods east of Interstate 35, which divides the city.

Police were warning people to be extremely cautious if they see something suspicious.

Laura McGinnis, who lives in the Travis Country neighbourhood in southwestern Austin, said Monday that Sunday night's attack is frightening but that it hasn't personally affected her yet. However, she said the bombing and three others elsewhere in the city this month make her wonder why the bomber hasn't been caught.

Adam McGinnis, who also lives in the neighbourhood, said he was reading on his back porch Sunday night when he heard what sounded like a large gun blast. He said he figured it must have been a transformer that blew and didn't realize it was a bombing until his wife told him Monday morning.

• TIMELINE | Massive investigation unfolding after 4 bombings in Austin

Austin police have said whoever was responsible was trying to send a message and should contact authorities to explain any motive.

"We are not going to understand that [message] until the suspect or suspects reach out to us to talk to us," Manley said.

Manley said police were also investigating the bombings as possible hate crimes.

Police get hundreds of calls

The first bombing on March 2 killed Anthony Stephan House, a 39-year-old black man. It ripped a hole in a home entrance wall and damaged the front door.

A bomb last Monday morning killed Draylen Mason, a 17-year-old African-American teenager and budding musician, and injured his mother, who is in her 40s but was not further identified. A few hours later, a third bomb injured the 75-year-old Hispanic woman, who has not been identified.

The victims in Sunday's explosion are white, Manley told ABC News on Monday morning.

Police have received more than 700 calls about suspicious packages since the three parcel bombs, but authorities have not found any that posed a security risk, Manley said.

A reward of $115,000 US has been offered for information leading to the arrest and conviction of the person or people responsible.